1. Field of the Invention
This invention relates generally to a thin film forming method and a thin film forming apparatus, and more particularly to a method and apparatus for forming a thin film on a surface of a substrate having a trench or an unevenness thereon, e.g., a semiconductor substrate.
2. Description of the Prior Art
The processes usually used to form a thin film on a surface of a substrate, such as a semiconductor, are classified broadly into Chemical Vapor Deposition (CVD) and Physical Vapor Deposition (PVD).
The CVD process induces a chemical reaction on the substrate surface or in gaseous phase to form a thin film on the substrate, and this process is used to form insulation films such as a silicon oxide film or a silicon nitride film. The PVD process forms a than film utilizing collision against a substrate of depositing materials generated in the gaseous phase, and this process is mainly used for metal film forming.
To achieve satisfactory results, VLSI device fabrication presently requires that a thin film be deposited within a trench formed in the substrate having an aspect ratio one or more (depth/width).
FIG. 22 is a sectional diagram showing a typical conventional plasma CVD process of the prior art (for example, J. L. Vossen & W. Kern, Thin Film Processes: Academic Press, 1978). In this process, an insulating film 53 is formed by depositing the deposit materials 52, which is in the solid phase generated in gaseous phase within a trench 51 of high aspect ratio formed on a substrate 50, such as silicon. However, the deposit material is deposited heavily on an edge 54 of the trench 51, thereby obstructing other deposit material from entering Toward a bottom 55 of the trench 51. Thus, a cavity 56 is formed within the trench 51 and there is a degradation of stage coating properties on the substrate surface.
To cope with the above problem, a process called bias sputtering process, which is one of the PVD processes, is employed (for example, T. Mogami, M. Morimoto & H. Okabaashi: Extended Abstracts 16th Conf. Solid State Devices & Materials, Kobe, 1984, p. 43). This method is to form an insulating film, such as a silicon oxide film, by physically sputtering the substrate surface with ions of argon, for example. In the application of this method, the sputtering makes it difficult to have much deposition on edges, as shown in FIG. 22, and promotes heavier deposition on the flat surface portions. Therefore, the problems of forming the cavity 56 and of the stage coating properties are reduced in comparison to the CVD process above.
However, as the deposit material in gaseous phase comes into The trench on a slant, it is difficult to achieve a good filling within the trench with an aspect ratio of one or more. This method actually has a low deposition velocity, which means a very low productivity, because of the competing reactions between the removal of the deposited film and the film deposition by physical sputtering. In addition, radiation damage is inevitable because the process is conducted in the plasma.
Recently, an ECR bias sputtering method (for example, H. Oikawa; SEMI TECHNOLOGY SYM, 1986, E3-1) was proposed to reduce the oblique incident element of the deposit material within the trench. This method lessens the above-mentioned problem of the oblique incidence of the deposition material within the trench even though the deposit material is in the solid state, but it is not a complete solution. Appropriate forming of a thin film with a trench of the high aspect ratio is still difficult.
Other than the processes described above, a method to form a silicon oxide film using thermal decomposition method a TEOS (R. D. Rung, T. Momose & Nagakubo; IEDM. Tech. Dig. 1982, p. 237) has been proposed. This method, as shown in FIG. 23(a), has a large surface movement rate of the deposit material, which makes cavity forming difficult, and realizes good stage coating properties. However, when an oxide film 57 having trenches formed by this method is cleaned, for example with diluted HF solution, the removal velocity of the oxide film 57 at the center of the trench 51 is extremely high, as shown in FIG. 23(b), and as a result, flat filling actually cannot be achieved. The reason seems to be the fact that the distortion of the oxide film grown from both sides of the trench remains around the center. It is thus considered extremely difficult to fill a trench with a high aspect ratio, even when a conformable thin film forming method is employed.
In FIG. 23(a), after formation of an oxide film with impurity as a solid phase diffusing source using the thermal CVD method etc., a thermal process may be applied to diffuse the impurity around the trench of the substrate. However, when comparing an oxide film formed on the side wall of the trench and that on a flat surface, the former has less impurity density, and a desired resistivity cannot be obtained with this method.